Vanillin can be manufactured by a number of synthetic methods. One of the most utilized methods which involves oxidative alkaline hydrolysis of spent sulfite liquor, is called the lignin process. Lignin is an amorphous polymeric substance related to cellulose that together with cellulose forms the woody cell walls of plants and the cementing material between those cell walls. In the lignin process vanillin is produced by a copper catalyzed, alkaline air oxidation of lignosulfonates which are the predominant material of fermented spent-waste liquors from sulfite pulp mills. A major by-product of this lignin process is acetovanillone. Prior to the present invention, acetovanillone was treated as an impurity of the lignin process for vanillin. Acetovanillone was separated and either discarded or used as a pharmaceutical intermediate, food preservative or sun-screening agent. Crude vanillin containing small amounts of acetovanillone as an impurity has also been available commercially, e.g. as a substitute for pure vanillin for many chemical purposes.
Attempts have been made to convert acetovanillone into vanillin; however, heretofore, the only method of doing so involved the use of nitrobenzene. Such a method was first reported in an article entitled "A New Vanillin Synthesis" by Henry O. Mottern, Canadian Journal of Chemistry, 56:2107-2108(1934). Unfortunately, nitrobenzene reduces to nitrosobenzene which in turn reduces to phenylhydroxylamine which reduces to aniline along with certain ortho and para azo benzenes. Such compounds are considered undesirable because of their carcinogenic behavior. Accordingly, the nitrobenzene oxidation of acetovanillone to vanillin has not been commercially adopted.
Other methods have also been tried. See, the Canadian Journal of Chemistry, 45:3009-3011(1967) which reports the use of various oxidizing agents, particularly cupric oxide, in the oxidation of acetovanillone. This method, however, produced very little, if any, vanillin.
It is an object of the present invention to produce vanillin from acetovanillone without the need to introduce nitrobenzene as an oxidizing agent.
It is a further object of the present invention to provide a method for recycling internal plant streams of the lignin process where such streams are rich in acetovanillone and to produce additional vanillin from said internal streams.
These objects and others are attained by a process which provides for oxidation of acetovanillone with either air or air enriched with oxygen in the presence of an alkaline material. Such oxidation is effected in a medium substantially devoid of nitrobenzene.